Considering you know the file's directory, its extension, and that it is the only one with this extension in this directory, you can simply use this mv :

mv /Users/xx/xx/*.csv /Users/xx/xx/test.txt

This will rename every .csv file in /Users/xx/xx/ (in your case, your single CSV file) into /Users/xx/xx/test.txt.

That would obviously be a problem if there was multiple .csv files since each would overwrite the previous one. If it could become a problem, you should either check the number of files which are matched by this glob and exit with error if it's greater than 1 or use a command that will only ever act on a single file such as the one suggested by RomanPerekhrest.